Bitumen is produced from oil sand such as is found in the Fort McMurray region of Alberta, Canada. The oil sand is mined and has been commercially recovered using hot water processes which generally require slurrying the oil sand with at least heated water or steam. The slurry is conditioned and the resulting froth separated from the solids portion for recovery of the bitumen therefrom.
The bitumen-containing froth is typically diluted with a hydrocarbon diluent, such as a naphthenic or paraffinic solvent, to reduce the viscosity and to aid in separating the bitumen from water and solids contained in the froth. Separation of the bitumen from the solids and water typically results in a bitumen-rich stream and a solvent-containing tailings feed stream.
In order to improve the economics of the bitumen recovery processes and the environmental impact of tailings disposal, solvent recovery apparatus and processes, generally known as tailings solvent recovery units (TSRU) are employed to recover the solvent from the solvent-containing tailings stream prior to disposal. The recovered solvent is typically recycled for use in the froth treatment processes. Conventional solvent recovery is typically effected by increasing the surface area of the solvent-containing tailings feed stream by creating a thin film of feed using internals such as shed decks or by retaining a pool of solvent containing tailings within the vessel for sufficient time, typically at least 5 to 15 minutes, to permit solvent release from the pool.
U.S. Pat. No. 6,712,215 to Scheybeler describes a solvent recovery vessel having pairs of nozzles located within the vessel. Each pair of nozzles is arranged horizontally in a 180-degree, opposed relationship. The nozzles have a first inlet for receiving solvent-containing tailings and a second inlet for receiving steam. A single outlet discharges a mixed steam/tailings stream. Adiabatic expansion of the steam, when contacted with the tailings, carries the discharge stream at high speed toward a centre of the vessel. The opposing orientation of the nozzles causes the discharge streams to collide at high speed forming small droplets in the vessel. The relatively larger surface area of the droplets facilitates release of the solvent from the liquid phase to the vapour phase. Shed decks located below the collision zone are heated using a steam sparger located beneath the shed decks and act to spread the droplets into thin layers thereon to facilitate heat transfer resulting in further solvent release. Another steam sparger is located in a lower portion of the column and further aids in stripping residual solvent from the liquid within the vessel. Pressure in the vessel is maintained at sub-atmospheric conditions through use of a steam eductor.
Canadian Patent 2,272,045 to Mocal Energy Limited et al (Syncrude) teaches introduction of froth treatment tailings into a near-atmospheric steam stripping TSRU vessel having a plurality of interior, vertically-spaced shed decks. The liquid tailings stream is evenly distributed over the shed decks creating a thin film of the liquid feed on an upper surface of the shed decks. Steam is introduced below the vertically-spaced shed decks and flows countercurrent to the tailings stream in order to vapourize the hydrocarbon diluents from the thin film of feed material. The vessel is preferably maintained at near atmospheric conditions (about 95 kPa).
Canadian Patents 2,587,166, 2,613,873 and 2,614,669 to Imperial Oil Resources Limited teach steam stripping TSRU vessels having internals, typically shed decks, with feed streams that contain asphaltenes. Both one-stage and two-stage processes are claimed. Steam is introduced below the internals or directly into a pool of liquids in the vessel and, in all cases, no agitation is utilized to mix the contents of the pool. At least a first TSRU vessel is operated at an absolute pressure between 100 kPa-200 kPa and at a temperature of between 75° C. and 100° C. A second TSRU is contemplated and is operated at a pressure lower than the first TSRU, being about 20-200 kPa and preferably 25-100 kPa and at a temperature of between 75° C. and 100° C. Additional TSRU vessels are contemplated to be added in series.
In the prior art vessels, steam is generally injected below the internals, such as shed decks and the like. The temperature of the internals therefore can only become as hot as the temperature of the steam being condensed at vessel operating pressures. The internals in the vessel therefore provide a relatively poor heat transfer surface, particularly where the feed contains asphaltenes which require significant heat to soften and release solvents trapped therein.
Further, in conventional TSRU's, the majority of the steam is condensed in contact with the tailings feed stream at the bottom of the vessel to transfer heat to the tailings stream. As a result, the partial pressure of the solvent is higher at the top of the vessel compared to the bottom of the vessel. The concentration of solvent vapour is also highest at the top of the vessel and therefore there is less impetus for mass transfer within the vessel.
There remains significant interest in the industry for apparatus and processes which result in more efficient and effective solvent recovery, particularly for feed streams which contain paraffinic solvents and asphaltenes resulting therefrom and to meet ever more stringent regulations regarding minimizing solvent loss and tailings disposal generally.